Welcome to Brainstoff from houstof works dot com, where smart happens him Marshall brain with today's question, how does a pressurized airplane cabin work? And what happens when that pressurization fails. This month, the world was treated to an unexpected surprise. It was the site of an airplane cabin with a big hole in it, open to the sky. We can imagine that this spectacle was even more unexpected to people
who were on the flight. If you saw photos or video of the whole you may have also been struck by how little there is between the inside and the outside of an airplane. There was a piece of plastic headliner on the inside of the plane, some insulation, and then a thin aluminum skin on the exterior of the plane. That's it. It brings up an interesting question, what is going on inside an airplane cabin when it's cruising at thirty three thousand feet. It turns out the passengers are
flying in something that vaguely resembles a space capsule. Let's take a look at how the space capsule works. The first thing to understand is that people dressed in normal clothing definitely cannot survive at thirty three thousand feet. This altitude is roughly the equivalent to standing at the summit of Mount Everest. If there were some way you could stick your arm out the window at thirty three thousand feet, the first thing you would notice is that it's incredibly
cold minus forty degrees fahrenheit or colder. The second problem is incredibly low air pressure. The pressure is so low that people would pass out very quickly from lack of oxygen. The air at that altitude and temperature is also extremely dry. So how are we able to sit in an airplane's comfy chairs at thirty three thousand feet feeling like we're sitting in someone's living room. The first thing that has
to happen is pressurization. The air it's sea level is about fourteen point seven p s i, or pounds per square inch. The pressure at thirty three thou feet, roughly six miles up, is approximately four p s i. Something has to be done to increase that pressure or people would quickly pass out from lack of oxygen at four p s I. Fortunately, the jet engines on the aircraft act like big air compressors. If you take a part of jet engine and look at it. It has four
main sections. At the front where the air is coming in, there is a compressor stage blades second air, and compress it. The fuel is injected into the compressed air stream and ignited in the combustion stage. That air expands greatly from the heat of combustion and flows through another set of blades,
turning them as it passes through. The energy from those turning blades is going to be transmitted forward by a shaft to turn the compressor blades, and then the exhaust gases flow out of the engine at high speed to create thrust to keep the airplane in the air. By creating and opening in the engine between the compression stage and the combustion stage, high pressure air can bleed out of the engine and feed into the cabin to pressurize it.
Because this air has just been compressed, it's hot. Therefore, the ventilation system on the plane will first cool it down using the extremely cold outside air that's already available to a comfortable temperature. The air pressure inside the plane is not sea level pressure. It's more like Denver pressure, where Denver is the mile high city. You can think of the airplane's cabin like a big pressurized tube that's been pressurized to about the air pressure that you'd experienced
in Denver. Now we have a cabin that is pressurized and warm, but because the outside air is so incredibly dry, some consideration has to be given to humidity. Fortunately, the plane is full of humidifiers. People give off moisture every time they exhale, and also through perspiration, So the dry air from outside is mixed with air already in the cabin and recirculated. The ratio of new air to existing air is typically fifty fifty. The recirculated air passes through
filters that remove any airboard particulates. The air in the cabin is still dry even after this recirculation process, but not nearly as dry as it could be. What happens if cabin pressurization fails. This can occur if the airplane skin ruptures or a window breaks. I've been on a flight where the copilot's windows simply cracked, and that was enough to depressurize the cabin. When that happens, the masks overhead will deploy and the pilot will immediately start descending
down to a safe altitude like eight thousand feet. The masks get their oxygen, not from pressurized tanks of oxygen they would be too heavy, but instead from a chemical reaction involving something like potassium chlorate. When heated, potassium chlorine gives off lots of oxygen, and a chemical oxygen canister like this is very light relatively speaking. So the next time you board an airplane take a moment to marvel
at what's happening. You'll be sitting in a chair at thirty three thousand feet, just like you might sit in your living room. An amazing amount of technology makes that possible. Be sure to check out our new video podcast, Stuff from the Future. Join how Stuff Work staff as we explore the most promising and perplexing possibilities of tomorrow the house Stuff Works. I Find app has arrived down at it Today on iTunes